Acute myocardial infarction (AMI) is a common cause of death for which effective treatments are available providing the condition is rapidly diagnosed. The modern diagnosis of AMI relies on the rise and fall of a specific serum biomarker accompanied by an appropriate circumstance such as chest pain or revascularization. In this accepted paradigm the diagnosis cannot be ruled in or ruled out without the definite presence or definite absence of a serum biomarker. The ideal biomarker of cardiac injury should be cardiac specific and released rapidly after myocardial injury in direct proportion to the extent of damage. Furthermore the biomarker should have a high sensitivity and specificity. (1) Several biomarkers of AMI have been described in the literature but only a few have found their way into routine clinical practice of which none are ideal. For example, CK-MB starts to increase 4 to 8 hours after coronary artery occlusion and returns to baseline within 2-3 days. (2) However its use is limited by its presence in skeletal muscle and normal serum and by sensitivity of the assay to interference causing some to question its utility. (3) Myoglobin is another cytoplasmic protein found in cardiac and skeletal, but not smooth muscle. It is released even earlier, within 1-2 hours of AMI and peaks within 5 to 6 hours. (2) Unfortunately, any injury to skeletal muscle also causes elevated levels of myoglobin reducing specificity. Fatty acid binding proteins (FABP) are small (15 kDa) cytoplasmic proteins expressed in all tissues with active fatty acid metabolism. Amongst the nine proteins, heart-specific FABP (H-FABP) is found in heart, but also kidney, brain, skeletal muscle and placenta. (4) Following acute MI H-FABP can be detected within 20 minutes and peaks at 4 hrs considerably faster even than CK/CK-MB in the same patient cohort. Although H-FABP concentrations in normal plasma are low they are known to rise non-specifically during physical exertion (without a troponin rise), kidney injury and stroke. (5)
The most specific and sensitive cardiac proteins released after acute myocardial infarction are cardiac troponin I and T. Both troponin I and T are released slowly, peaking approximately 18 hours after MI, and remain elevated for 7 to 10 days. (2) This slow release is likely the result of their relatively inaccessible cellular location compared to CK-MB, myoglobin and H-FABP. Troponins regulate the physical interaction of actin and myosin and thus are found almost entirely associated within the crystalline structure of the sarcomere of striated muscle cells. (6) The troponin complex is composed of 3 forms: I, T and C. Troponins I and T exist as cardiac specific isoforms with epitopes that differ from the corresponding skeletal isoforms. In addition, the absent, or extremely low, normal circulating levels of troponin provides the greatest dynamic range of any of the currently available biomarkers. (7) Whilst there is no doubt troponins have revolutionized the detection and management of patients with AMI (8) they do have disadvantages. The slow release of troponin delays diagnosis and the initiation of specific treatments that could salvage heart tissue in those in whom it is raised. Similarly, patients in whom it is absent, and who are ultimately reassured and discharged, are admitted to hospital unnecessarily. Furthermore, the persistence of troponins limits their utility in the diagnosis of reinfarction. It is therefore widely accepted that there is a need for new biomarkers that can diagnose AMI earlier during its natural history and/or that have a short plasma half-life allowing use in diagnosis and quantification of reinfarction. The purpose of this study was to use the platform of the crystalloid perfused mouse hearts to perform a systematic proteomicanalysis of the coronary effluent after minimal AMI in order to identify new potential biomarkers.